The present invention relates to the use of polymer dispersions for the dry lubrication of hydrogenated polymer materials such for example rubbers, for example EPR, EPDM, SBR, or plastics, metals, glass having an improved friction coefficient. More specifically said polymer dispersions are aqueous dispersions of cationic fluorinated oligourethanes obtained from functional (per)fluoropolyethers (PFPE) which optionally can be formulated with other fluoropolymers.
Generally the dry lubrication is selected when it is necessary to maintain for long time the lubricant effect among the surfaces in contact, avoiding theyr damage by the friction. By this treatment therefore it is possible to maintain during the time the properties of said surfaces. The dry lubrication has a remarkable importance when the materials in contact are rubber or plastic materials, and therefore they generally have poor wear resistance. An example of dry lubrication is that which occurs on the rubber parts used as fittings for the mobile glass safety or not of cars for reducing the friction and noise. Said fittings, that besides friction are exposed to the action of the light and atmospheric agents, must be lubricated so that the rubber surface is not worn and maintains during the time the mechanical properties.
Dry lubrication systems for improving the friction coefficient are known in the prior art. Formulations based on polysiloxanes and silicon rubber, crosslinkable at 150xc2x0 C. with hydrosoluble polyamines, able to form coatings having a thickness from 1 to 3 microns on rubbery surfaces EPDM type and to give a friction coefficient from 0.4 to 0.7 (1 Kg, 100 mm/min) are described in EP 761,791. The drawback of said formulations is that the friction coefficient is not satisfactory and besides thy are bicomponent systems wherefore the pot life is not high. In C.A. 131 300652t No. 22 1999, page 676, the Abstract of the patent JP 11 291769 is reported, wherein a coating for EPDM rubber is described, obtained by mixing with a polyurethane prepolymer based on polytetramethylenglycol (PTMG) a fluororesin in powder and a perfluoropolyether. Tests carried out by the Applicant with hydrogenated polyurethane prepolymers and a fluororesin in powder (PTFE) have shown that the obtained friction coefficient values are unsatisfactory. Compositions based on photocrosslinkable silicones (polyorganosiloxanes having epoxy functionality), particles of crosslinked silicone resin, having sizes from about 0.5 to 12 microns, able to give coatings having a low friction coefficient, are described in EP 903,385. Said formulations require the use of UV lamps for the photocrosslinking after the coating application and drying. From the industrial point of view this kind of crosslinking is an additional cost of the plant. In U.S. Pat. No. 5,115,007 formulations in solvent (toluene) for obtaining coatings on EPDM rubbers, formed by blocked isocyanic prepolymers formulated together with crosslinking agents and silicone oils, are described. The application of said formulations in solvent raises problems connected to the environmental impact and to the use in the work environment of organic solvents. Besides the obtained coatings have an high thickness, in the range 11-16 microns. U.S. Pat. No. 4,676,995 describes formulations is organic solvent, based on modified polyamides containing brominated and chlorinated groups on amidic nitrogen. Said formulations are applied on rubbers EPDM type in solutions at 4% of dry product in methylene chloride, forming coatings having a low friction coefficient. Also for said formulations in solvent it can be repeated what noticed above for the use of organic solvents in this kind of compositions. Similar formulations, that use organic solvents, are described in U.S. Pat. No. 4,720,518 and besides contain fillers such silica, PTFE powder, etc.
The need was felt to carry out dry lubricant treatments on rubbers, plastics, metals and glass, applying aqueous polymer systems, so as to avoid the use of organic solvents and to reduce the environmental impact, to eliminate the problems connected to the use of said solvents in the work environment, obtaining a dry lubricant layer showing the following combination of properties:
dynamic friction coefficient (ASTM D 1894-78)  less than 0.4, preferably xe2x89xa60.3,
high adhesion to the treated surface (ASTM D 3359-87),
high resistance to photooxidative degradation,
high rubbing-resistance,
high resistance to water absorption,
dry lubricant layer having a reduced thickness, in the range 0.1-5 microns, preferably 1-3 microns.
The Applicant has unexpectedly and surprisingly found that it is possible to obtain said combination of properties by applying aqueous dispersions of thermocrosslinkable fluorinated oligourethanes having a cationic functionality.
An object of the present invention is the use of fluorinated polyurethanes thermally crosslinkable for obtaining coatings having an improved friction coefficient for the dry lubrication of rubbers, plastics, metals, glass, said crosslinkable polyurethanes obtainable from aqueous dispersions of cationic oligourethanes based on branched and thermocrosslinkable (per)fluoropolyethers (PFPE), said cationic oligourethanes having a number average molecular weight lower than or equal to 9,000, determined by vapour pressure osmometry and formed by the following monomers and macromers:
a) aliphatic, cycloaliphatic or aromatic polyisocyanates, having NCO functionality, determined by titration with dibutylamine-HCl (ASTM D2572), higher than 2, preferably in the range 3-4;
b) bifunctional hydrogenated monomers having the two functions chemically different from each other (hetero-functional monomers) having the general formula:
XOxe2x80x94(CR1AR2A)bxe2x80x94Y0xe2x80x83xe2x80x83(Ib)
xe2x80x83wherein:
R1A and R2A, equal to or different from each other, are H, aliphatic radicals from 1 to 10 carbon atoms,
b is an integer in the range 1-20, preferably 1-10,
XO=XAH with XA=O, S,
Y0 is a salifiable, anionic or cationic function,
when in the formula (Ib) XO=OH, b=1, R1A=R2A=H and
Y0 is a hydrophilic group preferably having formula
xe2x80x94CH2Oxe2x80x94(CH2xe2x80x94CH2O)nTxe2x80x94CH3xe2x80x83xe2x80x83(Ib1)
wherein nT is an integer in the range 3-20; and one or more of the following compounds:
c) bifunctional hydroxylated (per)fluoropolyethers (PFPE diols) having number average molecular weight in the range 400-3,000, preferably 700-2,000;
e) monofunctional hydroxyl or carboxylic (per)fluoropolyethers (e0) or monofunctional hydroxyl (per) fluoroalkanes (exe2x80x2), said compounds (e0) and (exe2x80x2) having number average molecular weight in the range 300-1,000, preferably 400-800.
and optionally the following compounds:
d) hydrogenated monomers with which it is possible to insert a crosslinkable chemical function in the oligourethane, said monomers having formula (Ib), wherein R1A, R2A, b and XO are as above and Y0, is selected from the following functional groups: 
xe2x80x83wherein
RIB=H, CH3;
Rx is a C1-C5, preferably C1-C3, saturated alkyl;
dI) hydrogen-active compounds, able to form with the NCO functions bonds which are stable to hydrolysis but thermolabile, said compounds known as blocking agents of the NCO group, and selected from those known in the prior art such as for example ketoximes, for example methylethylketoxime, phenols and mono-, di-alkyl substituted phenols wherein the alkyl chain contains from 1 to 8 carbon atoms, pyrazol, caprolactam, ethylmalonate, acetylacetone, ethylacetoacetate.
The preferred composition of the invention comprises a)+b)+c), optionally e).
Preferably the amounts of the components a)xe2x88x92c), monomers and macromers which constitute the oligourethanes according to the present invention are the following:
component a) polyisocyanate: 10-70% by weight based on the total of the dry oligourethane, preferably 20-40% by weight;
component b) ionic heterofunctional hydrogenated monomer: the amount by weight based on the total of the dry oligourethane is calculated in connection with the molecular weight of the monomer, taking into account that the moles of component b) are in a ratio with the moles of the NCO groups of component a) comprised between ⅓:1 and ⅔:1 [(component b) moles: NCO moles)];
component c) PFPE diol: the amount by weight is in connection with the molecular weight of the macromer c), taking into account that the moles of the hydroxyl groups of component c) are in a ratio with the moles of the residual free NCO groups (the difference between the total ones and those combined with b)) comprised between 3 and 1.1, preferably 1.5 and 1.1; component c) can also be absent, and in this case component e) is present;
when c) is absent, the total amount by moles of components e)+d)+dI) is in a 1:1 ratio with the moles of residual NCO (the difference between the initial total moles of a) and the moles of a) reacted with b)), and component e) must be present in an amount of at least 30% by weight based on the dry product;
when component c) is present in the formulation, the total moles of the components d+dI+e, are in a percentage comprised between 0 and 90%, preferably 0 and 60% with respect to the moles of component b).
The aliphatic, cycloaliphatic or aromatic polyisocyanates indicated in a) are those available on the market and can for example be polyisocyanurates, biurets, adducts of the following diisocyanates: hexamethylendiisocyanate HDI, isophoron diisocyanate IPDI, toluendiisocyanate TDI, diphenylmethandiisocyanate MDI, hydrogenated diphenylmethandiisocyanate H12-MDI.
Preferred compounds are Vestanat T1890(copyright) (IPDI trimer) (Huls), Tolonate(copyright) HDT-LV (HDI trimer) (Rhone-Poulenc).
With heterofunctional monomer, a monomer having a functional group at each end of the chain is meant, said functional groups being different from each other.
The heterofunctional hydrogenated monomers indicated in b) wherein preferably in the XAH function XA=O, preferably have the following formula of structure: 
wherein T is SO3H, COOH, or a tertiary amino group NRxe2x80x2NRxe2x80x3N, wherein Rxe2x80x2N and Rxe2x80x3N, equal or different, are linear or branched C1-C6 alkyl; Rxe2x80x21A and Rxe2x80x31Axe2x80x2, equal or different, are hydrogen or linear or branched C1-C4 alkyl; n1A is an integer in the range 1-10, preferably 1-4. The monomers b) of formula (1A) wherein T is a tertiary amino group, such as for example dimethyl-aminoethanol, diethyl-aminoethanol, dimethyl-aminopropanol, diethyl-aminopropanol, are preferred.
The bifunctional (per)fluoropolyethers indicated in c) have one or more of the following units statistically distributed along the chain: (C3F6O), (CFYO) wherein Y is F or CF3, (C2F4O), (CR4R5CF2CF2O) wherein R4 and R5 are equal to or different from each other and selected from H, Cl, and a fluorine atom of the perfluoromethylene unit can be substituted with H, Cl or (per)fluoroalkyl, having for example from 1 to 4 carbon atoms.
The preferred compounds of c) are the following with the perfluorooxyalkylene units statistically distributed along the chain:
aI) xe2x80x94(C3F6O)mxe2x80x2(CFYO)nxe2x80x2xe2x80x94
wherein the units (C3F6O) and (CFYO) are perfluorooxy-alkylene units statistically distributed along the chain: mxe2x80x2 and nxe2x80x2 are integers such as to give the above mentioned molecular weights, and mxe2x80x2/nxe2x80x2 is in the range 5 and 40, nxe2x80x2 being different from 0; Y is F or CF3; nxe2x80x2 can also be 0;
bI) xe2x80x94(C2F4O)pxe2x80x2(CFYO)qxe2x80x2xe2x80x94(C3F6O)txe2x80x2xe2x80x94
wherein pxe2x80x2 and qxe2x80x2 are integers such that pxe2x80x2/qxe2x80x2 ranges from 5 to 0.3, preferably from 2.7 to 0.5 and such that the molecular weight is within the above limits; txe2x80x2 is an integer with the meaning of mxe2x80x2, Y=F or CF3; txe2x80x2 can be 0 and qxe2x80x2/(qxe2x80x2+pxe2x80x2+txe2x80x2) is equal to {fraction (1/10)} or lower and the txe2x80x2/pxe2x80x2 ratio ranges from 0.2 to 6;
c1) xe2x80x94CR4R5CF2CF2Oxe2x80x94
wherein R4 and R5 are equal to or different from each other and selected from H, Cl; the molecular weight such to be within the above limits, and a fluorine atom of the perfluoromethylene unit can be substituted with H, Cl or (per)fluoroalkyl, having for example from 1 to 4 carbon atoms; the end groups of the bifunctional (per)fluoropolyethers c), said end groups being equal to or different from each other, are of the HO(CH2CH2O)x0CH2xe2x80x94 type wherein x0 is an integer in the range 0-4, preferably 0-2; in the preferred compounds x0=0.
These (per)fluoropolyethers are obtainable by known processes. See U.S. Pat. No. 3,665,041, U.S. Pat. No. 2,242,218, U.S. Pat. No. 3,715,378 and EP 239,123.
The preferred heterofunctional monomers among those indicated in d) have the same above formula (1A) of the component b) wherein Rxe2x80x21A, and Rxe2x80x31A and n1A are as above defined and T is instead selected from the groups that in component d) are at the place of the Y0 function, the OH group of the formula 1A can optionally be substituted with a SH group.
The process for preparing the oligourethanes is described in the European patent application EP 00112141.7 in the name of the Applicant.
Under the application conditions used for the crosslinking of the formulations, the thermal crosslinking of the cationic oligourethane is obtained without the hydrogen-active compounds belonging to the above class dI), when the heterofunctional monomers component b) are aminoalcohols having the above general formula (1A). When said aminoalcohols are present in the oligomer, it is possible indeed by thermal treatment to restore the NCO function of the urethane bond with the hydroxyl group of the heterofunctional monomer component b) as above defined, lowering the amount of the ionic groups present on the oligourethane structure (aminoalcohol evaporates at the crosslinking temperature) which is under crosslinking. Without to be bound to any theory, this fact could explain the high resistance to water and the maintenance of the improved friction coefficient in the time, as it is shown in the following Examples, of the coatings obtained with the oligourethanes as above defined.
When component e) is formed by monofunctional hydroxyl or carboxylic (per)fluoropolyethers (e0), they comprise one or more of the (per)fluorooxyalkylene units above indicated for the PFPE diol component c).
Preferred (e0) compounds are for example the following ones, wherein the following units are statistically distributed along the chain:
IB) Axe2x80x2O(C3F6O)m(CFYO)nxe2x80x94
wherein Y is xe2x80x94F, xe2x80x94CF3; Axe2x80x2=xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7, xe2x80x94CF2Cl, C2F4Cl; the C3F6O and CFYO units are randomly distributed along the (per)fluoropolyether chain, m and n are integers, the m/n ratio is xe2x89xa72. These compounds are obtainable by hexafluoropropene photooxidation according to the process described in GB 1,104,482;
IIB) C3F7O(C3F6O)mxe2x80x94
wherein m is a positive integer, wherein the number average molecular weight is the one above indicated. These compounds are obtainable by ionic telomerization of hexafluoropropene epoxide: see for example U.S. Pat. No. 3,242,218;
IIIB) (C3F6O)m(C2F4O)n(CFYO)qxe2x80x94
wherein Y is equal to xe2x80x94F, xe2x80x94CF3; m, n and q, different from zero, are integers such that the number average molecular weight is that indicated for the component e). Said compounds are obtainable by photooxidation of mixtures of C3F6 and C2F4 by the processes described in U.S. Pat. No. 3,665,041;
the reactive monofunctional end group is of the Tb(CH2CH2O)x0CH2xe2x80x94 type wherein x0 is an integer between 0 and 4, preferably 0 and 2, more preferably x0 =0, Tb is OH.
Alternatively, when the component e0 is a monofunctional carboxylic (per)fluoropolyether, the monofunctional end group is xe2x80x94CF2xe2x80x94COOH.
When component e) is formed by monofunctional hydroxyl (per)fluoroalkanes (exe2x80x2), said compounds preferably have the formula:
(RfI)pIQxe2x80x94OHxe2x80x83xe2x80x83(exe2x80x2)
wherein:
RfI is a C3-C30, preferably C3-C20 fluoroalkyl radical;
pI is 1 or 2;
Q is a bivalent C1-C12 aliphatic or C6-C12 aromatic linking bridge; Q can optionally contain heteroatoms such as N, O, S, or carbonylimino, sulphonylimino or carbonyl groups; Q can be not substituted or it is linked to substituents selected from the following: halogen atoms, hydroxyl groups, C1-C6 alkyl radicals; Q preferably does not contain double or triple bonds and is saturated; preferably Q is selected from the following divalent radicals: xe2x80x94CH2xe2x80x94, xe2x80x94C2H4xe2x80x94, xe2x80x94SO2N(R5)C2H4xe2x80x94, xe2x80x94SO2N(R5)CH2CH(CH3)xe2x80x94, xe2x80x94C2H4SO2N(R5)C4H8xe2x80x94, R5 being H or a C1-C4 alkyl.
When the monofunctional PFPE component e0 has a carboxylic end function, the bond formed with xe2x80x94NCO is of amidic type instead of urethane type.
The oligourethanes according to the present invention have a number average molecular weight preferably in the range 2,000-9,000. The number average molecular weight can be determined by methods known in the prior art, such as for example vapour pressure osmometry VPO. Suitable solvents for carrying out said determinations are the fluorinated ones such as trifluoroethanol, or also non fluorinated such as for example ethyl acetate.
The starting compositions containing the oligourethanes for obtaining the coatings to be applied on materials such as rubbers, plastics, metals and glass for the use according to the present invention are under the form of monocomponent aqueous dispersions. The formulation stability is over 12 months.
The dry content is in the range 1-70%, preferably 10-30% by weight.
Said aqueous dispersions usable according to the present invention are obtained by salifying the oligomer in organic solvent adding an acid, which can be organic or inorganic, dispersing the organic mixture in water and eliminating the organic solvents by evaporation.
The Applicant has found that the lubricant properties of the polymers of the present invention can be furtherly improved by formulating the oligourethanes in aqueous dispersion in mixture with other different fluoropolymers.
The total percentage by weight of fluoropolymers which are added to the aqueous dispersion of the oligourethane ranges from 0 to 30%, preferably from 1 to 10%.
TFE (co)polymers can for example be used, preferably using the corresponding concentrated polymerization latexes, preferably stabilized by mixtures of non ionic and cationic surfactants. Non ionic surfactant is for example Triton(copyright) X100. As cationic surfactants cetyl trimethylammonium bromide can be mentioned.
The TFE (PTFE) homopolymers or the TFE copolymers with one or more comonomers containing at least one unsaturation of ethylene type can be used both in powder and under the form of latex, preferably they are used under the form of latex or dispersion. The latexes or dispersions of TFE fluoropolymers are formed by homopolymers of tetrafluoroethylene (TFE) or its copolymers with one or more monomers containing at least one unsaturation of ethylene type. The amount of comonomer can for example range from 0 up to 3% by moles, preferably from 0.01 to 1% by moles; the average particle sizes range from 20 to 400 nm, preferably from 80 to 300 nm. These aqueous dispersions from 180 to 400 nm are available on the market as Algoflon(copyright) D60 and they are obtainable by the conventional polymerization processes in aqueous emulsion.
The dispersions of the homopolymers and copolymers of TFE wherein the average particle sizes range from 20 to 80 nm, preferably 20-60 nm, can also be formed by TFE thermoplastic copolymers, preferably copolymers containing from 7 to 27% by weight of hexafluoropropene; copolymers containing from 0.5 to 18% by weight, in particular from 2 to 10% by weight of one or more perfluoroalkylvinylethers and/or fluorinated dioxoles, preferably selected from methyl-, ethyl-, propylvinylether, 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TDD), perfluoro-2,2-dimethyl-1,3-dioxole (PDD).
Mixtures of dispersions containing both particles from 190 to 400 nm and those from 20 to 80 nm can also be used. The ratio by weight between the dispersions with the component having higher particle sizes with respect to the dispersions with the component having lower particle sizes can range from 99:1 to 1:99.
The nanometric aqueous dispersions between 20 and 80 nm are obtainable by the polymerization process in microemulsion described in patent application EP 969,055 in the name of the Applicant.
It has been found that the addition to the oligourethane dispersions of (per)fluoropolyethers having (per)fluorinated end groups and kinematic viscosity at 20xc2x0 C. in the range 30-300 Centistokes (3xe2x88x9d300xc3x97107m2/s) (ASTM D 445), allows to decrease the rigidity of the crosslinked fluorinated oligourethane. The non reactive PFPE amount is generally in the range 0-20%, the upper limit is the one that does not lead to the formation of two or more phases but only one aqueous phase of the polymers of the invention dispersed in water is obtained. The (per)fluoropolyethers having (per)fluorinated end groups are added to the organic solution containing the oligourethane, which is then salified and dispersed in water, lastly removing the organic solvent used in the oligourethane synthesis. To the so obtained formulation it is possible to add TFE fluorinated polymers as above defined, preferably under latex form.
The catalyst chemical classes both for the synthesis of the oligourethanes of the invention and for unblocking the thermolabile groups are well known in the prior art. The organometal or aminic ones commonly used for the polyurethane synthesis can be mentioned; those soluble or dispersible in water such as for example: dibutyltindilaurate or dialkyltin salts having mobile bridging bonds Snxe2x80x94S, for example Fastcat(copyright) 4224 are particularly preferred; diethylentriamine, ethylendiamine, Jeffamina(copyright) T 403 (triamine linked to a polyoxypropylene chain), N-ethyl-ethylendiamine, diazobicycle octane, etc., can be mentioned as amines. The catalyst is added in concentrations generally ranging from 0.1 to 5% by weight and preferably from 0.5 to 1%. For the synthesis of the oligourethanes of the invention it is preferable to use a small amount of catalyst, while the catalyst for unblocking the thermolabile groups is added to the water formulation even just obtained. As said the stability of this monocomponent formulation is very high wherefore it is industrially very useful for the production cycles.
The formulations of the invention are easily applicable also at concentrations of 40% in dry product, since the viscosity is low also at these concentrations.
Additives such as catalysts as above indicated, crosslinking co-catalysts, ionic and non ionic, also fluorinated, surfactants, photostabilizing additives, fillers and pigments can optionally be added to the finished water formulation. Photostabilizing additives are for example UV adsorbers, for example hydroxybenzophenone, hydroxybenzotriazole derivatives, etc., HALS (hindered amines), such as for example derivatives from tetramethyl-piperidine, etc. Additives pigment type are for example metal oxides such as titanium dioxide, iron oxides, mixed oxides of Ni, Co, Zn, Ti, or Cr, Cu or Fe, Ni, Cr, Mn, cobalt aluminates; organic pigments such as derivatives from anthraquinone, quinacridone, tetrachloroisoindolinone, diketoperylene, phthalocyanines, etc.), and fillers such as for example silica, polyamides having 20-100 s sizes, glass spheres zeolites type.
Other additives which can be added are thixotropic agents, dispersants, preferably polymers, for pigments and fillers; extending, anticissing, antifoam additives, etc.
The dispersion containing the oligourethanes, formulated with the above additives, can be diluted with water up to oligourethane concentrations, expressed as percentages by weight, in the range 1-50%, preferably 10-45%.
The dispersions according to the present invention are applied by conventional methods, for example by spray, roll or dipping on the above substrata.
The coating thickness is in the range 0.1-5 microns, preferably 1-3 microns.
The invention coatings are effective also at said very low thickness values and this represents an advantage since the bulk mechanical properties, such as elongation, modulus, bending of the substratum are not substantially modified.
Preferably the structural surface in hydrogenated polymer material (rubber or plastic) on which the dispersion of the oligourethanes must be applied, is previously subjected to a treatment, such as for example plasma or corona treatment, to generate on the surface of the polymer material reactive polar sites, to improve both the wettability with the aqueous dispersion and the adhesion to the surface after crosslinking of the oligourethane. Depending on the type of the atmosphere (air, oxygen, nitrogen) in which the plasma or corona treatment is carried out, various reactive groups such as for example xe2x80x94OH, xe2x80x94COOH, xe2x80x94COxe2x80x94, xe2x80x94NHRxe2x80x94 functions can form on the surface of the hydrogenated polymer material. These functions are able to cause interactions both of physical and chemical nature with the cationic groups and the xe2x80x94NCO groups which are generated in the oligourethane during the thermal unblocking reaction during crosslinking, assuring a very good adhesion to the hydrogenated polymer substratum.
The use according to the present invention is carried out, as said, by applying the dispersions containing the oligourethanes on the surfaces of the above materials by the previously described methods, subsequent drying and crosslinking at temperatures of 130xc2x0 C. or higher for a period of time in the range 1-30 minutes, in connection with the used temperature. For example, at a temperature of 180xc2x0 C. crosslinking is completed in 5 minutes.
The following Examples illustrate the invention, without limiting the scope thereof.